This invention relates to a spray drying process for the production of Phase II sodium tripolyphosphate.
Sodium tripolyphosphate ("STPP") exists in two major crystalline forms, known as Phase I (or Form I) and Phase II (or Form II). Each form, and more usually mixtures of the forms, can be prepared by heating (calcining) essentially dry phosphates other than tripolyphosphates, or by spray drying an orthophosphate or pyrophosphate liquor. More Phase I material is formed if the process temperature is maintained above 450.degree. C. More Phase II material is formed at temperatures below 450.degree. C. The transition temperature between the two forms is about 420.degree. C. The two crystalline forms have different properties and industrial detergents and cleansers are often formulated around these properties. In many applications, the two forms are not interchangeable.
Spray drying is the preferred process for producing STPP because it is fast and can be operated as a single step and on a large scale. The alternative process, calcining of essentially dry feed material in rotary calciners, can be used to produce high purity (at least 90% by weight) of either form but the process takes hours as compared with minutes and even seconds in the spray drying process. Spray drying has been readily adapted to production of high purity Phase I material but until the present invention, attempts to adapt existing one-step spray drying processes and equipment to production of Phase II material have not been successful. Thus, U.S. Pat. No. 3,438,725 discloses a one-step spray drying process for producing substantially pure Phase II STPP. However, the process depends on use of wet process acid and careful control of the amounts of sulfate and metallic impurities. Other spray drying processes have resulted in heavy caking on the walls of the dryer due, in part, to the lower temperatures required for Phase II STPP production.
Calcining processes provide no effective guidance to production of high purity (at least 90%) Phase II STPP by spray drying. For example, U.S. Pat. No. 2,920,939 describes a process for calcining essentially dry phosphate mixtures of tripolyphosphates substantially free of impurities with accelerated rates of conversion by adding at least 1% by weight of crystalline alkali metal tripolyphosphate to the phosphate mixture. The patent states that STPP, and Phase II material in particular, can be produced practically completely free from impurities by incorporating into the reaction mixture an amount of crystalline Phase II material which is several times as much (for example, 8 or 10 times) as nontripolyphosphate reactants in the mixture. The patent also demonstrates that the incorporation of smaller amounts (e.g., 5 wt. %) of Phase I material or STPP hexahydrate is far more effective than the incorporation of the same amounts of Phase II material for improving the production of Phase II STPP. However, even the preferred practice gives only about 82% conversion to Phase II product.